1. Field of the Invention
The present invention relates to an improvement made to devices for the heat treatment of yarns in motion in order to impart to the latter certain shrinkage and/or voluminosity and/or elasticity properties.
In the description which follows the conventional expression of "heater" will be employed to denote such a device which, of course, can be employed both for treating continuous, mono- or multifilament yarns and for fibre threads.
2. Description of the Prior Art
In all processes for the treatment of yarns in motion requiring passage through a heater, the chief problem which arises is that of fast heat transmission, it being necessary for the heat to enter uniformly throughout the yarn and in the same manner over the whole length of yarn. In fact, as is known, the temperature of the treatment and its uniformity have a very great influence on the qualities of the yarn produced.
It is well known that the heat treatment varies as a function of the treated substance, the yarn count and its speed of passage inside the heater. Thus, it can be easily seen that the core of a fine yarn would be reached more quickly than that of one of a high count. Similarly, it is known that a yarn cannot be treated above a certain ceiling temperature, or else it degrades. As a result, since this question of heat exchange is very important in the textile field, very many solutions have been envisaged to solve it, these solutions making use of the three great principles of heat exchange, namely convection, radiation and conduction.
Besides this problem of good heat transmission there is also that of maintaining the yarn under a specific tension which is a function of the treatment that is carried out. Thus, for example, in the case of a drawing operation, optionally combined with a texturing operation, the yarn must be kept under a maximum tension whereas, on the other hand, when a yarn shrinkage is to be carried out, the tension must be reduced.
Finally, there is also the problem of the increase in the production rates on a textile machine, which have increased from a few tens of meters per minute about thirty years ago to a thousand meters, or even more at present, these high speeds generating phenomena such as vibrations, which are difficult to control.
Among the solutions proposed to obtain a very good uniformity in heat transmission and also to facilitate the problems of temperature control from one position to another on a machine in order that the whole production should be homogeneous, it has been envisaged for a very long time to carry out heat treatments which could be qualified as being "sequential" by producing heaters over the length of which a number of separate heat treatment zones follow one another. Among these solutions there may be mentioned those forming the subject of French Patent 1,204,634, in which two successive treatments are carried out inside the same heater, one during which the yarn is subjected to a temperature which is considerably higher than that of the normal temperature to which the yarn must be heated, this being done for a period such that the latter is not damaged, this first stage being followed by an equilibrium treatment at a conventional temperature. Although such a solution is temptingly attractive owing to the fact that it makes it possible to reduce considerably the length of the heaters for a given speed of travel, it requires equipment which is of complex design and difficult to use.
A similar solution is described in French Patent 1,216,847 and its certificate of addition 76861.
In the case of other types of treatment it may be desirable to have a different heat exchange sequence, for example to have a slow and uniform rise in bringing the yarn up to temperature, followed by a more considerable thermal shock, or to combine such stages in a different manner.